I agree with others who said that the site looks good, but it may be better to show a grid of themes immediately on the first page, and then using the filters would narrow what is shown. Currently users need a minimum of 3 clicks before they see any of the themes (in worst case they need 9 clicks when they specify each 4 category). That's maybe not a huge issue but it could be improved even just by showing some themes after the first category is selected, then narrow the list after the other filters are changed.

Another thing i noticed is the loading animation before showing the themes. It may be a little slow because I'm viewing it from the EU and the servers are in US, but it could be improved a little if the next template would be preloaded after the current one is loaded, or even when the filters are already selected but the "Show templates" button isn't clicked yet.

For advertising you could try the more design-oriented ad networks: http://vandelaydesign.com/ad-networks/

And in your AdWords campaign you could create ads with more specific keywords that are linking to a page with prefilled filters, for example: users who come from a "Responsive Wordpress themes" ad would have to click only on the "Show themes" because the filters are already set for them on your page. And also more specific keywords could cost less than general more competitive keywords (but specific keywords has less volume so you need to create more of these keyword combinations to get more traffic).